An ion discharge apparatus basically creates plasma from neutral atoms and accelerates the ions in a desired direction. A stream of discharged ions can be used to irradiate a target, or even to provide ion propulsion to spacecraft. Thus, high-power ion propulsion utilizing ion engines or thrusters have been created with a design life to extend for a significant period of time.
The performance of an ion thruster depends chiefly on the design and performance of the ion extraction grids. Fundamentally, the maximum beam current that the grids can extract for a fixed specific impulse is limited by space-charge effects, electron backstreaming, and electrical breakdown (arcing) between the grids. These effects themselves are related to the hole alignment between the screen, accelerator, and decelerator grids, and to the grid-to-grid separation distances.
The problems inherent in increasing the thrust density of ion engines are grid erosion and thermal distortion which changes the grid separation distances. Grid erosion, due to ion sputtering of the grid surfaces by discharge chamber or charge-exchange ions, becomes more severe as the thrust density increases because there are more ions to erode the grids. Thermal distortion is due to nonuniform heating, and the resulting thermal expansion, of the grid electrodes because of radial and grid-to-grid temperature gradients.
For several reasons, inert gases have replaced mercury as the propellants of choice for proposed interplanetary and earth-orbital ion propulsion systems. Erosion rates on ion engine discharge components, however, are expected to be greater with inert gas propellants than the corresponding rates with mercury. This is due in part to the higher sputter yields of the inert gases as compared to mercury. In addition, discharge and beam currents in ion engines operated on inert gases will be greater compared to ion engines that operate on mercury propellant, for the same thrust level. These combined effects will limit the operating life of inert-gas ion engines.
Presently, state-of-the-art grids are fabricated from molybdenum sheets. To mitigate the grid distortion problems, the grids are dished by hydroforming; for example, a J-series engine grid is dished approximately 2.0 cm over the 30-cm diameter. With this technology molybdenum grids have been fabricated up to 50 cm in diameter.
However, there are limits to dished molybdenum grid technology. For example, it is difficult to hydroform the grids uniformly across the entire diameter of the grid, which leads to a nonuniform grid gap. In addition, the hydroforming process may cause grid-to-grid hole misalignment. The finite coefficient of thermal expansion for molybdenum results in thermal distortion which becomes more severe as the grid diameter is increased.
Thus, in applications of grid members that may be subject to significant heat and erosion, the prior art is still seeking to optimize the performance of ion discharge apparatus.